As techniques for formation of color hard copies, various processes such as the heat-sensitive transfer process, electrophotographic process and ink jet process have been widely investigated. The heat-sensitive transfer process is superior to the other processes in various points, since the maintenance and operation of the equipment is easy and the equipment itself and supplies for the equipment are low-priced. The heat-sensitive transfer process can be grouped into two types; one is a system where a heat-sensitive transfer material having a hot-melting ink layer as formed on a base film is heated with a thermal head so as to melt the ink and images or letters are recorded on a copying sheet with the transferred molten ink, and the other is a system where a heat-sensitive transfer material having a heat-transferring dye-containing colorant layer as formed on a base film is heated with a thermal head so as to transfer the dye to a copying sheet by, e.g., sublimation or diffusion. The latter heat-transferring system is especially advantageous for recording full-color images with high image quality, since the amount of the dye to be transferred to the copying sheet may be varied by controlling the energy to be applied at the thermal head and therefore gradation recording is possible.
However, the heat-transferring dyes usable in this system have various limitations and there are known only a few dyes which satisfy all the necessary characteristics and are satisfactorily usable in the system.
The necessary characteristics of the dyes are, for example, such that the dyes have preferable spectral characteristics for color reproduction, the dyes are heat-transferred with ease, the dyes are fast to light and heat, the dyes are resistant to various chemicals, the sharpness of the images to be formed from the dyes is hardly lowered, the images to be formed from the dyes are hardly re-transferred, the dyes may be produced with ease, and heat-sensitive transfer materials having the dyes may be produced with ease. Accordingly, development of heat-transferring dyes which satisfy the necessary characteristics has been desired in this technical field.
Hitherto, various heat-sensitive transferring dyes have been proposed. Above all, azomethine dyes described in JP-A-60-239289, JP-A-61-268493, JP-A-62-191191, JP-A-63-91287, JP-A-64-63194, JP-A-1-176591, JP-A-1-176590, JP-A-63-113077, JP-A-64-1591, JP-A-1-176592 and JP-A-63-205288 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") have relatively excellent characteristics. However, these still have a problem that the spectral characteristics thereof are not satisfactory for sufficient color reproduction.
Furthermore, they were not satisfactory in light-fastness and heat-fastness. The improvement in light-fastness and heat-fastness has been further desired.
The following developing agent is described in Photographic Science and Engineering, vol. 8, number 3, May-June (1964). ##STR2##
The absorption coefficient of the azomethine dye generated from the developing agent is described in Table IIIb of Photographic Science and Engineering, vol. 8, number 3, May-June (1964). It is described that the azomethine dye prepared from the developing agent has an absorption coefficient larger than those prepared from the other developing agents. However, since the developing agent is a remarkably instable compound, it was not practical that the developing agent is used as a synthetic intermediate to prepare a dye.